LIMULUS POLYPHEMUS. 



497 



INDEX LETTERS TO PLATE XXVI. 



EXPLANATION OF PLATE XXVI. 



Fig. 49. Longitudinal section through the middle of the second lobe of an 

 adult nephridium. The section was 5 /j. thick and was stained in borax carmine 

 and Lyon's blue. The median side of the lobe is at the right. The lobe is com- 

 posed of four distinct regions or layers, each of which has a characteristic structure. 

 X 16. (i) The outer portion is formed of very large granular cells (^-c.) (see 

 Fig. 50) most abundant at the lateral end of the lobe on both dorsal and ven- 

 tral sides. Two smaller groups are shown on the median side. Small nerve 

 fibers penetrate this tissue. (2) A.c. A dark layer which surrounds the lobe, 

 except for a short distance on its median ventral side. It is composed of small 

 cells with fine granules around the periphery. They are probably hollow cells 

 {/i.e.) which are uniting end to end (see Fig. 51). (3) g-.i. A faintly stained layer 

 inside of /i.e. and surrounding the entire lobe. It is composed of small tubules 

 which are lined with large granular cells (see Fig. 52). (4) The larger part of 

 the lobe is contained in this tubular portion (i.p.) (see Fig. 53). (5) c.(. The col- 

 lecting tubular portion is similar to (4), except that the tubules are larger. They 

 connect the small tubules of each lobe with the collecting tubes of the stolon. It 

 is shown on the dorsal side of the figure and near the median end of the lobe 

 (see Fig. 54). 



Fig. 50. An enlarged drawing through the large cells which surround the 

 lateral end of Fig. 49 {g:c.). They are filled with small granules, which did not 

 stain in either borax carmine and Lyon's blue, or Delafield's haematoxylin and 

 eosin. Several nuclei are often found in the same cell. Nerve fibers penetrate 



